1. Technical Field
The present invention relates to an LCC (Limited Current Circuit) of a digital inverter for an LCD backlight, and more particularly, to an LCC of a digital inverter for an LCD backlight that can perform a stable protection function while meeting a standard specification.
2. Discussion of the Related Art
Generally, an inverter used for an LCD backlight uses a high voltage. The inverter has an international standard that is set so that current higher than a predetermined level cannot flow through a human body when a user is contacted with the inverter by taking the user's stability into account.
For this, in general, it is checked whether an Limited Current Circuit (LCC) of an inverter is properly operated in such a manner that a human body is set to a noninductive resistance of 2KΩ and the noninductive resistance is contacted to a second side of a transformer. When the noninductive resistance of 2KΩ is connected to a second side coil of the transformer that outputs a high voltage, the LCC is implemented using the phenomenon that a voltage value or current value output from the transformer become smaller than an output (reference voltage or reference current) at the time of normal operation. Specifically, when a striking process starts, the inverter gradually increases a voltage for driving a lamp. In a case where a noninductive resistance is electrically connected to the inverter, current also flows in the noninductive resistance. However, a voltage value higher than the international standard may flow through the noninductive resistance at any time. In this case, the voltage output to the transformer becomes lower than the output value of the transformer at the time of normal operation. That is, if the output value of the transformer is lower than the reference value (reference voltage or reference current), a protection circuit of the LCC is operated to shut down the inverter. For example, as shown in FIG. 1, the a comparator 12 outputs a shutdown signal to a shutdown enable latch 15 when an output voltage is lower than a reference voltage Vref by comparison between an output voltage of a transformer 11 for a master, whereby the inverter is shut down.
In the same manner, a comparator 14 outputs a shutdown signal to the shutdown enable latch 15 when an output voltage is lower than the reference voltage Vref by comparison between an output voltage of a transformer 13 for a slave and the reference voltage Vref, whereby the inverter is shut down. For reference, an LCC check is classified into a striking LCC and an operating LCC. The striking LCC is contacted to a noninductive resistance before an inverter operation and operated, and the operating LCC is contacted to a noninductive resistance during an inverter operation. That is, a separate LCC is operated before and after the operation of the inverter, respectively. FIG. 2 shows an output current waveform of a striking LCC of a related art inverter. An X-axis is time, and a y-axis is a voltage output from a transformer. As shown in FIG. 2, if it is asserted that an output value of the transformer is lower than a reference value (reference voltage or reference current), the LCC is operated, to thus abruptly shut off the voltage output from the transformer, thereby preventing a current higher than the international standard from flowing through a human body.
Recently, cases of applying a method of implementing the aforementioned LLC of analog control type to an inverter of digital control type are increasing. In the digital control type, a microcontroller unit (MCU) perform an arithmetic operation according to the order of a series of programs, and performs an LCC check after the completion of the operation. That is, the LCC does not operate while the MCU is carrying out the operation.
However, in the LCC requiring a high speed process, if the method of implementing the aforementioned LCC of analog control type is applied to the digital control type inverter, there is a problem that a delay time is generated according to the amount of operation of the MCU and current beyond the international standard may flow through a human body. For example, the MCU may perform an interrupt processing routine. In this case, a delay time may be generated since another operation cannot be performed. In addition, an operation delay of the MCU may be generated by an environment such as a low temperature and a high temperature, which renders difficulties in ensuring the reliability of a protection circuit such as an LCC.